Radiation sources in charged particle accelerators



Aug. 20, 1957 G. c. BALDWIN 2,803,767

RADIATION SOURCES IN CHARGED PARTICLE ACCELERATORS Filed Sept. 30. 1952 Fig.1.

TlME-VARYING VOLTAGE SOURCE Inventor: George C. Baldwin,

His Attorney.

United States Patent RADIATION SOURCES IN CHARGED PARTICLE ACCELERATORS George C. Baldwin, Albany, N. Y., assignor to General Electric Company, a corporation of New York Application September 30, 1952, Serial No. 312,265

4 Claims. (Cl. 313--62) The present invention relates to charged particle accelerator apparatus and, more particularly, to radiation sources in charged particle accelerator apparatus.

Apparatus for accelerating charged particles by means of magnetic induction effects is shown and described in United States Patent Nos. 2,394,071, 2,394,072 and 2,394,073, all of which were patented February 5, 1946 by Willem F. Westendorp and assigned to the assignee of the present invention. Such apparatus can comprise a core of magnetic material ineluding a pair of opposed, rotationally symmetrical pole pieces which define a toroidal gap wherein an evacuated container is positioned. The core is excited by means of windings that are energized by a source of time-varying voltage to produce a time-varying magnetic flux which links an equilibrium orbit within the evacuated container and a time-varying magnetic guide field which traverses the equilibrium orbit. Charged particles, e. g. electrons injected along the equilibrium orbit from an electron gun positioned adjacent to the orbit within the region of influence of the time-varying magnetic guide field, are accelerated to high energy levels by the time-varying magnetic flux during a great number of revolutions while the time-varying magnetic guide field constrains the particles to follow paths along the equilibrium orbit. After acceleration to a desired energy level, the charged particles can be diverted from the equilibrium orbit to a target for the generation of X-radiaion.

In the utilization of magnetic induction accelerator apparatus and other forms of accelerator apparatus employing a time-varying magnetic guide field, it is often desirable to generate an X-ray beam which has been produced by charged particles of a single energy. An X-ray beam of this nature necessarily must be obtained from a thin target, i. e. a target which does not reduce appreciably the energy or the particles traversing it, because otherwise the X-rays produced would contain those created by particles of many differing energies. With apparatus of the above-described forms, however, the diversion of the charged particles from the equilibrium orbit to a thin target does not provide the desired results inasmuch as the charged particles make repeated traversals of the thin target, whereby the thin target is effectively transformed into a thick target and the X-ray beam generated is one resulting from multi-energetic particles.

It is therefore a principal object of the present invention to provide a means of obtaining an X-ray beam generated by charged particles of single energy from charged particle accelerator apparatus of the forms described.

A further object of the present invention is to provide true thin target X-radiation with charged particle accelerator apparatus of the forms described.

Another object of the present invention is to provide a means for obtaining two X-ray beams during the same acceleration cycle of charged particle accelerator apparatus of the forms described, one of the X-ray beams being true thin target radiation and the other being thick target radiation.

According to one aspect of the invention, a thin target is positioned radially inward from the equilibrium orbit in charged particle accelerator apparatus. A thick target is placed at a defined azimuth with respect to the thin target and radially inward from the outer edge of the thin target. Charged particles diverted from the equilibrium orbit first strike the thin target to produce true thin target X-radiation. After the particles have traversed the thin target, they continue and strike the thick target before they have had an opportunity to re-traverse the thin target. By positioning the thick target at a defined azimuth with respect to the thin target, the charged particles are assured of striking the thick target before they can re-traverse the thin target; furthermore the thick target radiation thus obtained is produced from a circle of confusion or focal spot having minimum radial and vertical extent, whereby a collimated beam of thick target radiation is obtained in addition to the true thin target radiation.

The features of the invention desired to be protected herein are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a simplified, partly sectionalized view of magnetic induction accelerator apparatus suitably embodying the invention;

Fig. 2 is a sectionalized view of a portion of the envelope of Fig. 1 with thin and thick targets positioned according to the invention;

Fig. 3 is a section view taken along lines 33 of Fig. 2; and

Fig. 4 is a section view taken along lines 4-4 of Fig. 2.

Referring particularly now to Fig. 1, there is shown in exemplary fashion magnetic induction accelerator apparatus suitably embodying the invention. The apparatus comprises a magnetic core 1 which can be laminated to minimize the generation of eddy currents therein. Core 1 includes laminated, rotationally symmetrical, opposed pole pieces 2 and 3 having generally outwardly tapered pole faces 4 and 5 for the provision of a magnetic guide field traversing an equilibrium orbit O, as will be more fully described hereinafter. Coaxial with pole pieces 2, 3 and disposed between pole faces 4, 5 is an evacuated annular container or envelope 6 of dielectric material, which provides within its interior an annular chamber 7 wherein charged particles can be accelerated. The central portions of pole pieces 2, 3 are terminated respectively by fiat surfaces 8, 9 between which are disposed laminated metallic disks (not shown) and dielectric support spacers 10, 11. The metallic disks serve the purpose of reducing the reluctance of the magnetic path in the region between surfaces 8 and 9.

Magnetic core 1 can be excited from a suitable source of time-varying voltage 12 connected as indicated to series-connected energizing windings 14, 15 surrounding pole pieces 2, 3. To minimize the current drawn from source 12, energizing windings 14 and 15 can be resonated by power-factor-correcting capacitors 16. Within chamber 7 adjacent to equilibrium orbit O and also within the region of influence of the time-varying magnetic guide field existing between pole faces 4, 5 during operation of the apparatus, there is provided a charged particle source 17 which is supported from a hermetically-sealed side arm 18 of envelope 6. More detailed illustration and description of electron gun structure suitable for present purposes can be found by reference to the above-mentioned patents or by reference to the United States Patent No.

3 2,484,549 of J. P. Blewett, patented October 11, 1949, and assigned to the assignee of the present invention.

It is well understood by those familiar with magnetic induction accelerator apparatus that energization of windings 14, 15 by the source of time-varying voltage 12 results in a time-varying magnetic flux which traverses magnetic core 1 and pole pieces 2, 3 to provide a timevarying magnetic flux that links equilibrium orbit O and a time-varying magnetic guide field that traverses the locus of equilibrium orbit O and the vicinity thereof between pole faces 4, 5. Electrons emitted by gun 17 at a desired timed instant near zero in the cycle of magnetic flux and field variations are continuously accelerated during the acceleration portion of the cycle as they execute repeated revolutions along and about equilibrium orbit 0. As a consequence, the injected electrons can be caused to assume energies of many millions of electron volts and then can be automatically diverted from the equilibrium orbit by means of pulsatingly energized orbit shift coils 19, 20 to produce X-radiation in a manner which will be more fully described hereinafter. Means including circuits for arranging the proper timed injection and subsequent diversion of the charged particles from the equilibrium orbit at or near the end of the accelwhere A is the total change in flux linking the equilibrium orbit, R0 is the radius of the equilibrium orbit and B0 is the flux density of the time-varying magnetic guide field at the equilibrium orbit. The condition specitied by this relationship may be realized by making the reluctance for one unit area of cross section of the magnetic path of the time-varying flux greater by an appropriate amount at the equilibrium orbit than its average reluctance for one unit area of cross section within the orbit.

The fulfillment of the foregoing condition, however, only assures stable acceleration for those charged particles which are injected tangentially to their instantaneous" circles or orbits. The instantaneous circle or orbit is the circular orbit along which a charged particle started at the proper position with the right energy will travel in a time-constant, radially symmetric magnetic field. With a time-varying magnetic flux as above specified, the loci of the instantaneous circles of all the charged particles approach and eventually essentially coincide with the equilibrium orbit during the latter portions of the acceleration cycles. Consequently, meeting the foregoing condition does not take into consideration the requirements for stable acceleration of charged particles which tend for one reason or another to deviate from their respective instantaneous circles or to deviate from the equilibrium orbit when their respective instantaneous circles coincide therewith. Nevertheless, by arranging the spatial distribution of the timevarying magnetic guide field in the vicinity of the equilibrium orbit as specified by the following relationship, both radial and axial focusing forces which tend to constrain deviating particles to their respective instantaneous circles or to the equilibrium orbit can be provided:

tile H L: id 0! (log r) H dr (2) til) where H is the intensity of the time-varying magnetic guide field in the vicinity of the equilibrium orbit, r is the radius of a particular point under consideration and n is a parameter having a value lying between zero and one. The outwardly directed taper of pole faces 4 and 5 as illustrated in Fig. 1 enables the utilization of the condition set forth in Equation 2. It is apparent from Equation 2 that the parameter n is a measure of the rate of decrease of the time-varying magnetic guide field with radius. Both radial and axial focusing forces exist if U a l. For a uniform field, n:() and no axial focusing of the particles can take place. In a field inversely proportional to the radius, there are no radial focusing forces. Since both radial and axial focusing forces are required to secure collimation of the particle beam during acceleration. the foregoing limits are placed upon the selected value of the 11.

After the charged particles have been accelerated to a. desired energy level under the foregoing conditions, they must be diverted from the equilibrium orbit to permit useful utilization of the energy which has been imparted to them. As has been stated above, diversion of the charged particles from the equilibrium orbit can be accomplished by supplying a properly timed pulse of current to orbit shift coils 1? and 20. The application of the current pulse to the orbit shift coils modifies the magnetic induction throughout the stable region surrounding equilibrium orbit O and causes the charged particles to spiral very slowly inwardly or outwardly from the equilibrium orbit, depending upon the direction of the flux generated by the orbit shift coils with respect to the flux generated by the windings 14, 15. During their travel away from the equilibrium orbit the charged particles can be considered as following paths tangent to their respective instantaneous circles the radii of which are gradually decreasing or increasing as the case may be.

According to the present invention both true thin target X-radiation and thick target X-radiation may be efficiently obtained from charged particle accelerator apparatus during the same accelerating cycle by first causing the charged particles to traverse a thin target and subsequently causing the charged particles to strike a thick target before they have had an opportunity to retraverse the thin target. Referring specifically now to Figs. 2, 3 and 4, there is shown a thin target 21 and a thick target 22, both of which are positioned adjacent to and on the same side of equilibrium orbit 0. Thin target 21, which may comprise a thin (e. g. 0.005 inch) strip of material such as tungsten, copper, beryllium, aluminum, etc., is supported by a bent rigid rod 23 that is hermetically sealed into the wall of envelope 6 as illustrated. Rod 23 is extended downwardly shortly after it enters chamber 7 in order that charged particles will not impinge thereupon during their acceleration along equilibrium orbit 0. Thick target 22, which may comprise a relatively thick (e. g. 0.050 inch) plate of a material such as tungsten, copper, etc., is adjustably supported from a rigid rod 24 which is introduced into chamber 7 through a hermetically sealed bellows 25. The portion of rod 24 extending transversely of envelope 6 is likewise removed from the vicinity of equilibrium orbit O in order to avoid premature collision of the charged particles thereupon. The inner edge of thin target 21 is positioned at essentially the same radius as or more radially outward than the outer edge of thick target 22 so that charged particles spiraling inwardly from equilibrium orbit O first strike thin target 21. Preferably, the radial extent of thin target 21 is made equal to the decrement of particle orbit radius resulting from particle traversal of the thin target, as will more clearly appear presently.

Now it will be understood that when orbit shift coils 19, 20 are energized with a pulse of current to cause the charged particles to spiral inwardly after they have been accelerated to a desired energy level as described in the aforementioned Kerst Patent No. 2,394,070, the

charged particles first strike thin target 21. According to the invention, thin target 21 is so selected that it produces primarily a conversion of some of the energy of the charged particles into X-radiation and, in addition, a conversion of some of the particle energy into ionization and excitation in a manner which will be more fully explained hereinafter. As a result of their energy loss in traversing thin target 21, the charged particles move inwardly much more rapidly, thus facilitating their interception by thick target 22 before they have had an opportunity to re-traverse thin target 21. Thick target 22 is positioned farther from equilibrium orbit O and at a defined azimuthal spacing with respect to thin target 21 in order that substantially the entire charged particle beam can be intercepted by thick target 22 at a circle of confusion having minimum radial and vertical extent, whereby a collimated beam of thick target radiation is obtained in conjunction with the aforementioned true thin target radiation. The considerations involved in the selection of the proper position of thick target 22 with respect to thin target 21 will be discussed presently.

When charged particles such as fast electrons pass through target foil such as thin target 21, several processes occur. First, some of the energy of the charged particle is converted to radiation such as X-radiation. This process chiefly involves interaction with the coulomb fields of the target nuclei and can be determined by the following relation:

where W1 is the quantity of energy radiated in millions of electron volts, W is the energy of the charged particle, N is the thickness of the target element in rnols per square centimeter and Z is the atomic number of the material of the target element. Secondly, some of the energy of the charged particles is lost by ionization and excitation of the atoms of the target element. This process principally involves collisions with atomic electrons and may be calculated from the following equation:

Wi=3.6NZ (4) where W1- is the energy lost by ionization in millions, of electron volts. Thirdly, the charged particles striking the target element are multiply scattered. This process is the charged particle deflection accumulated as the result of many successive collisions, mainly with the nuclear fields in the target. It transforms an initially collimated beam into one with a Gaussian distribution in space, the root-mean-square angle of scattering being As had been mentioned above, the present invention contemplates the utilization of the radiation process in thin target 21 to generate true thin target radiation which is produced by charged particles having a single energy. The loss of particle energy represented by the thin target radiation generated, coupled with the ionization loss in thin target 21, causes the radii of the charged particles to decrease abruptly, and thick target 22 is positioned to intercept the particles before they can re-traverse thin target 21 and destroy the true nature of the thin target radiation. The successful employment of the invention depends upon the use of the radiation and ionization processes in thin target 21 to the practical exclusion of the scattering process. The above equations show that the scattering process decreases with higher energies of the charged particles, hence it is preferable that the charged particles be accelerated to relatively high energies before they are diverted to thin target 21.

After the charged particles have struck thin target 21 and have passed therethrough, their trajectories are of course affected by the restoring forces of the abovedescribed, time-varying magnetic guide field. Essentially none of the charged particles is on its respective instantaneous circle, hence each oscillates about its own instantaneous circle. The path followed by each particle about its own instantaneous circle has a radial frequency equal to 1 -;1 f and an axial frequency equal to VII), where f is the equilibrium orbital revolution frequency. According to the present invention, thick target 22 should be positioned in azimuthally spaced relationship with respect to thin target 21 such that target 22 will intercept the inwardly deflected charged particles before they can re-traverse thin target 21 and will also intercept the beam where it has a focal spot of minimum radial and axial extent.

It can be demonstrated that a charged particle scattered from its instantaneous circle by thin target 21 in a direction having an angle 3 with the tangent to the instantaneous circle will have oscillation amplitudes X and y at azimuth 0 as follows:

The first azimuth, 61, where x and y are equal produces a first circular focal spot. To determine the azimuth 01 for a selected value of the parameter n, one must solve the relation:

Accordingly, when the charged particle beam is deflected radially inwardly from the equilibrium orbit to strike thin target 21, positioning of thick target 22 approximately at the azimuthal angle 0 defined by Equation 8 produces thick target radiation from a target of minimum extent and optimum geometry. Since Equations 6 and 7 are sinusoidal functions, other values of 6 will satisfy the condition that x=y; however it can be illustrated that 0 must always have a value less than 720 to produce the X-ray beams according to the invention. Moreover, it can be shown that, with thick target 22 located approximately at one of these defined azimuthal angles, substantially all of the charged particles deflected by thin target 21 are intercepted in less than two revolutions after passing through thin target 21. Even though some of the charged particles may proceed for more than one revolution before they are intercepted by thick target 22, their oscillatory motion prevents their retraversal of thin target 21 since, for any permitted value of n, they cannot refocus at the original focal spot upon thin target 21 in less than two revolutions. As an example of the proper positioning of thick target 22 to secure the advantages of the invention, Equation 8 shows that for n=% thick target 22 should be positioned at an azimuth approximately mid-way between 350 and 416, i. e. 28, beyond thin target 21 for an assumed clockwise acceleration of the charged particles, as is illustrated in Fig. 2.

From the foregoing description it is readily appreciated that the present invention makes possible the generation of true thin target radiation in orbital charged particle accelerator apparatus. Moreover, thick target radiation can be produced in a separate beam and during the same cycle with the true thin target radiation. Thin target radiation is very desirable in such applications as cancer therapy while thick target radiation finds many useful applications in radiography. The present invention is not limited to utilization in conjunction with accelerator apparatus which employs magnetic induction phenomena alone, but can also be used with synchrotron apparatus such as that disclosed in United States Patent No. 2,485,- 409, patented October 8, 1949, by Willem F. Westendorp and Herbert C. Pollock and assigned to the assignee of the present invention. Further application of the present invention can be made in connection with non-ferromagnetic accelerating apparatus, e. g. the apparatus disclosed in United States Patent No. 2,465,786, patented March 29, 1949 by J. P. Blewett and assigned to the assignee of the present invention. In general, the present invention has application in all types of orbital charged particle 7 accelerating apparatus having a time-varying magnetic guide field essentially satisfying the conditions of Equation 2.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Charged particle accelerator apparatus comprising an enclosure defining a stable accelerating region containing an equilibrium orbit, means for establishing a time-varying magnetic guide field traversing said region and essentially satisfying the relation d (log 1') where H is the intensity of the time-varying magnetic guide field within the stable accelerating region, r is the radius of a particular point under consideration, and n is a parameter having a value lying between zero and one, a thin target within the enclosure having the outer edge thereof positioned radially inward from the equilibrium orbit, and a thick target positioned radially inward from the equilibrium orbit and within the enclosure the outer edge of said thin target but not substantially inward from the inner edge of said thin target, the azimuthal separation of said targets being approximately determined by the relation where 9 is the azimuthal angle between the two targets and has a value less than 720.

2. Apparatus as in claim 1 in which the outer edge of said thick target is at substantially the same radius as the inner edge of said thin target.

3. Charged particle accelerator apparatus comprising an enclosure defining a stable accelerating region containing an equilibrium orbit, means for establishing a time-varying magnetic guide field traversing said region, a thin target within the enclosure and having the outer edge thereof positioned radially inward from the equilibrium orbit and having both the properties of causing substantial energy conversion to X-radiation and substantial energy loss to ionization whereby charged particles diverted inwardly from the equilibrium orbit strike said thin target to produce a beam of X-radiation and particles having smaller instantaneous circles, and a thick target within the enclosure and having the outer edge thereof positioned radially inward from the equilibrium orbit and the outer edge of said thin target but not substantially inward from the inner edge of said thin target, the azimuthal separation of said targets being approximately determined by the relation where 6 is the azimuthal angle between the two targets and has a value less than 720 and n is a parameter defined by equation d (log H) d (log 1) where H is the intensity of the time varying magnetic guide field within the stable accelerating region, r is the radius of a particular point under consideration, and n is a parameter having a value lying between 0 and 1, where said particles having smaller instantaneous circles strike said thick target to generate a second beam of X-radiation from a defined focal spot.

4. Apparatus as in claim 3 in which the outer edge of said thick target is at substantially the same radius as the inner edge of said thin target.

References Cited in the file of this patent UNITED STATES PATENTS 1,137,964 Goddard May 4, 1915 1,645,304 Slepian Oct. 11, 1927 1,876,049 Forde a a1. Sept. 6, 1932 

